We are investigating the role of hypersensitivity to viruses by measurement of virus specific IgE antibody against viruses isolated from and detected in SIDS cases and by searching for histologic and biochemical evidence of hypersensitivity reactions. Measurements include: (1) virus specific immunoglobulins in serum and bronchial washings; (2) total histamine content of lung samples; and (3) study of tissue mast cells.